The invention relates generally to noise reduction techniques, and more particularly to techniques for reducing correlated low frequency noise in a complementary metal oxide semiconductor (CMOS) amplifier.
Data acquisition circuits are used in a wide range of applications requiring high quality data acquisition and processing. For example, in the field of medical imaging, imaging panels may detect the impinging radiation and convert them into measurable electrical charge through sensors. A data acquisition circuit may then read the electrical charge from the sensors in the imaging panel for subsequent conversion into digital data and image processing.
The first stage of a data acquisition system is typically a low noise amplifier, whose main function is to provide enough gain to overcome the noise of subsequent stages and transform charges into voltage for further processing in some applications. Aside from providing this gain, a LNA should add as little noise as possible and should consume as little power as possible. Additionally, in a multi-channel data acquisition system, the noise contributed by the LNA should not be correlated while power consumption and die size should be minimized. To accomplish this, a single ended amplifier is typically used in every channel instead of the traditional differential amplifier. To further reduce noise, power and area, a common bias circuit is used for all the single ended amplifiers. However, this approach causes any noise in the bias circuit to be correlated across all the channels sharing the bias circuit. Specifically, the correlated low frequency noise such as flicker noise is a nuisance since in the imaging domain, a human eye can average through other broadband noise sources and highlight the offending low frequency noise source. This causes objectionable artifacts in the acquired images.
For example, in systems such as a digital X-ray panel, a large number of sensors (e.g., photodiodes) are multiplexed into a single low noise amplifier (LNA) and reset. When the switch connecting the amplifier to a pixel is opened, it samples the low frequency noise onto the pixel. This is indistinguishable from signals that are generated from an X-ray exposure after the reset. Further, since a large number of photodiodes sampled the low frequency noise, the signal from all the photodiodes have this correlated component that a human eye can easily discern.
Current techniques for minimizing the correlated low frequency noise include use of bipolar CMOS (BiCMOS) or junction field effect transistor (JFET) based amplifiers, differential amplifiers, or a separate bias for each of the channels or amplifiers instead of common bias. However each of these techniques has one or more limitations. For example, BiCMOS or JFET amplifiers are expensive thereby making the detector circuit costly. Alternatively, the use of differential amplifiers increases the power requirement and other noises. Moreover, the use of separate bias circuits per channel increases the power and area requirement and is expensive.
It is therefore desirable to provide cost effective and efficient low noise amplifiers for data acquisition with minimal correlated low frequency noise interference.